How to solve the five major problems of LUN storage

After storage administrators have mastered the basic principles of managing logical unit numbers (LUNs), they need to solve more complex problems related to LUN storage, including resource management, multipath I/O related to Windows deployment, and configuring hot backups. Here are five related issues of LUN storage, see how expert Brien Posey explains to storage administrators.

　　1. Can LUNs be thinly provisioned?

　　Storage administrators often create LUNs that are larger than they actually need to avoid having to expand them later. This approach wastes a lot of disk resources. Oversized LUNs consume storage resources that could be used by other LUNs, even if the LUN to which they are allocated never actually uses them.

　　Windows 8 and Windows Server 2012 allow LUNs to be thinly provisioned, as long as the storage hardware is Windows 8 certified and supports thin provisioning. The benefit of thin provisioning LUNs is that it allows administrators to create LUNs that are larger than they actually need, but the underlying storage resources are only used when they are needed. The downside is that thin provisioning can overcommit storage resources and occasionally run out of physical storage space.

　　2. Does multipath I/O complicate Windows deployment?

　　Although multipath I/O is an important mechanism for ensuring storage availability, it can complicate the Windows installation process. If administrators attempt to install Windows Server 2012 on a LUN configured for multipath I/O, they often receive an error message stating that "Installation cannot create a new system partition or locate the current system partition."

　　There are two fairly simple ways to resolve this problem: You can temporarily configure the LUN for single-path I/O before performing the Windows installation, or initialize the boot LUN.

　　3. What is the maximum capacity of a LUN?

　　Depending on the software you use to create the LUN, you may be able to create a LUN that is larger than the capacity of the server can utilize. Therefore, when you decide how large to create a LUN, consider the limitations of your system. Here are the limitations you need to keep in mind when using Windows Server:

　　For traditional servers using the FAT file system, the maximum volume capacity is 4GB.

　　For traditional servers using the FAT32 file system, the maximum volume capacity is 32GB.

　　The maximum capacity of an NTFS volume on a master boot record disk (MBR disk) is 2TB.

　　The maximum capacity of an NTFS volume on a GPT disk is 16EB.

　　Currently, the maximum volume capacity recommended by Microsoft for creating NTFS is 16GB.

　　The maximum capacity of a traditional virtual hard disk file (VHD file) is 2040GB (less than 2TB).

　　The maximum size of a VHDX file is 16 TB.

　　4. Should the number of LUNs be limited?

　　Windows Server limits the number of LUNs that can be identified. If you use Fibre Channel connections, there are the following limitations:

　　For each host bus adapter (HBA), up to 8 buses can be defined.

　　Windows can recognize up to 128 targets per bus.

　　Windows recognizes up to 254 LUN IDs per target.

　　These numbers add up to mean that in Windows Server 2008 and above, the theoretical limit on the number of LUNs per HBA card is 260,996.

　　5. How can I use a hot spare disk?

　　Windows Server 2012 supports the use of hot spare disks. A hot spare disk is an additional disk that can immediately replace a failed disk. Windows allows you to assign hot spare disks to specific LUNs, and hot spare disks can also serve multiple LUNs. The storage subsystem can be configured to use hot spare disks automatically or manually. Hot spare disks are most useful when LUNs are mirrored or striped with parity because the system can automatically use the hot spare disk.

　　The hot spare disk must be the same size as the corresponding disk. It cannot be subdivided. For example, a 2TB hot spare disk cannot replace two 1TB disks.

　　It should be noted that if the hot spare disk configuration process fails, the LUN performance will be affected because the configuration process generates a large amount of disk I/O. When the hot spare disk configuration is completed, the LUN performance will return to normal.